6PPD in Tires and Naturally Occurring MOFs in Cable Bacteria: Lessons for Safer Materials and Bioelectronics

Podcast overview

The Chemistry World podcast delves into two cutting edge stories in chemistry and materials science: the search for safer tire additives to replace 6PPD and the discovery of living organisms that biosynthesize and use metal organic frameworks (MOFs) at the sea floor. The discussion ties together industrial chemistry, environmental health, and future materials design.

Key insights

• 6PPD protects tire rubber from ozone by reacting at the surface, but its breakdown product quinone is highly toxic to certain salmon species, raising regulatory and environmental concerns.
• Replacement strategies must preserve tire performance while avoiding new ecological risks, a challenge known as regrettable substitution.
• Researchers found cable bacteria produce nickel-based MOFs that form electrically conductive nanoribbons, enabling electron transport in deep-sea sediments, offering a potential blueprint for bio-inspired electronics.
• The podcast also reflects on the legacy of chemists like Christopher Ingold, whose concepts underpin modern reaction mechanisms.

Overview

In this edition of the Chemistry World podcast, host Mariana Kneppers guides listeners through two interlinked stories that sit at the crossroads of chemistry, environmental science, and materials engineering. The first story revisits a familiar tire additive, 6PPD, and examines why replacing it safely is more complex than it appears. The second story reveals a striking example of natural materials science: cable bacteria that build and use metal organic frameworks (MOFs) to shuttle electrons across centimeters of sediment. Taken together, these pieces illustrate how fundamental chemistry, environmental toxicology, and materials design intersect in the real world.

6PPD in tire chemistry and the search for a safe replacement

6PPD has been used in tires for roughly six decades to prevent rubber degradation, especially in the presence of ozone. Mechanistically, the preservative reacts with ozone at the tyre surface, protecting the rubber from cracking and maintaining performance. But diffusion through the solid rubber means the additive must also be mobile within the matrix to act where ozone is present. The byproduct 6PPD quinone can form when 6PPD degrades in the environment, and this compound has been shown to be deadly to at least some fish species, notably coho salmon, rainbow trout, and related populations. This toxicity problem has sparked regulatory moves in the US, Canada, and the EU, and has driven a push to identify safer replacements that preserve tyre performance while reducing environmental harm.

The discussion then turns to the chemistry of the 6PPD family. The core functional motif is a paraphenylenediamine unit, but variations with different side groups alter both performance and toxicology. Not all family members are equally toxic to fish, and European and Canadian authorities are proposing broader regulation of the entire class if the toxicology warrants it. A key industry challenge is uncertainty about how different homologues interact with various aquatic species; toxicology work for a wide range of candidates remains essential. Industry groups are testing dozens of potential replacements—some clovely PPDs and some non-PPD options—before narrowing to the most promising leads for more thorough toxicology profiling. In parallel, researchers consider the problem of “regrettable substitution”—replacing a known hazard with a similar compound that later reveals new risks. The tire industry must balance mechanical performance, diffusion within the rubber matrix, and safety across ecosystems, which may lead to some replacements being impractical despite initial appeal.

MOFs in cable bacteria: a natural metal organic framework

The podcast then shifts to a remarkable discovery in microbiology and materials science: cable bacteria, long filamentous bacteria that can transport electrons over centimeter-scale distances through sediments. Scientists have identified nickel-based MOFs that are biosynthesized by these bacteria, effectively acting as natural conductive cables. These MOFs are arranged into nanoribbons that conduct electricity with efficiencies comparable to leading synthetic MOFs and conductive polymers. The discovery challenges the long-standing view that MOFs are exclusively human-made and connects bioinspired design with practical electronic conduction in energy-poor environments.

How did this come about? Cable bacteria are known for their exceptional electrical connectivity in deep-sea sediments where light is scarce and energy must be harvested chemically from reduced species in the sediment. The nickel-MOF filaments appear to function as part of the electrical wiring that powers the bacteria’s metabolism, transporting electrons from sulfide oxidation in the sediment to the cellular sites where energy is generated. The researchers describe these MOFs as forming nanoribbons that resemble nanoscale ferredoxins in their role, yet composed of metal ions connected by dithiolene ligands, a motif familiar in certain natural metal cofactors and in synthetic MOFs. The work is framed as a form of convergent evolution in chemistry, where nature discovers motifs that humans have also explored for decades in materials science.

In terms of applications, the scientists suggest these naturally grown MOFs could inspire sustainable, biologically grown electronics, with potential routes to scale these structures or mimic their conductive networks synthetically. The structure of these nanoribbon MOFs, layered into extended ribbons, opens questions about manufacturing at larger scales and integrating bio-derived motifs into conventional device fabrication. The discovery adds a new dimension to MOF research by highlighting a living system’s capacity to produce and deploy a highly conductive network, which could influence future design of materials that combine high surface area with robust electrical transport.

Design lessons from biology and implications for future work

Beyond the specific stories, the podcast emphasizes how detailed understanding of structure, diffusion, and environmental interactions informs safer materials and innovative electronics. The MOF-based cables in cable bacteria suggest nature’s own approach to designing conductive networks in challenging environments, while the tire additive discussion reinforces the need for comprehensive toxicology and life-cycle analyses when selecting substitutes for hazardous substances. The episode emphasizes cross-disciplinary collaborations among chemists, toxicologists, engineers, and microbiologists to anticipate risks and harness bioinspiration for material design.

Historical perspective: Ingold and the language of reaction mechanisms

The episode closes with a look back at Christopher Ingold, a foundational figure in reaction mechanism theory. Ingold helped formalize ideas about electron movement in reactions, introduced revolutionary concepts such as curly arrows, and, together with collaborators, defined the SN1 and SN2 paradigms. His work, which clarified how electrons rearrange during chemical transformations, remains central to how chemists communicate mechanisms today. The discussion highlights how historical milestones in chemistry continue to shape modern thinking about complex processes such as those involved in 6PPD degradation and MOF biosynthesis.

Closing notes

The podcast invites listeners to explore chemistryworld.com for additional stories and to sign up for newsletters that capture the ongoing evolution of chemical sciences. This edition underscores how chemical safety and innovative materials research are deeply interconnected with environmental health and sustainable technology.